Apparatus and method for performing an outer loop power control in a mobile communication system

ABSTRACT

An outer loop power control apparatus and method in a mobile communication system are disclosed. If a first received frame has no errors and the raw BER of the first received frame is equal to or greater than a predetermined raw BER, a power control mode is changed to an adjustment mode. If a second received frame following the first received frame has no errors and the raw BER of the second received frame is less than the threshold raw BER in the adjustment mode, a target Eb/No is decreased by a product of a first step coefficient and a decrement unit. If the raw BER of the second received frame is equal to or greater than the threshold raw BER, the raw BERs of the first and second received frames are compared. If the raw BER of the second received frame is less than the raw BER of the first received frame, the target Eb/No is decreased by a product of a second step coefficient and the decrement unit.

PRIORITY

This application claims priority to an application entitled “Apparatusand Method for Performing Outer Loop Power Control in a MobileCommunication System” filed in the Korean Industrial Property Office onJan. 14, 2002 and assigned Serial. No. 2002-1979, the contents of whichare hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a mobile communicationsystem, and in particular, to an outer loop power control apparatus forsetting a target energy-to-noise ratio (Eb/No) adaptively according tochannel condition, and a method thereof.

2. Description of the Related Art

A CDMA (Code Division Multiple Access) mobile communication system iscomprised of Node Bs, RNCs (Radio Network Controllers), and UEs (UserEquipments). Each Node B performs downlink TPC (Transmit Power Control)and uplink TPC to minimize interference between UEs within its cell andinterference from other Node Bs. The downlink TPC controls the power ofa downlink signal transmitted from the Node B to a UE, and the uplinkTPC controls the power of an uplink signal transmitted from the UE tothe Node B. The downlink and uplink TPCs are commonly performed by openloop power control, closed loop power control, and outer loop powercontrol.

Those power control methods will be described below.

(1) Open Loop Power Control

A UE measures the propagation loss of a particular downlink channelsignal, for example, a P-CCPCH (Primary Common Control Physical CHannel)signal received from a serving Node B and controls its uplinktransmission power according to the propagation loss measurement, tothereby allow the Node B to receive an uplink channel signal reliably.

The P-CCPCH delivers information about the Node B and system information(SI) to UEs within the service area of the Node B. The P-CCPCH istransmitted with a constant transmission power and information about thetransmission power of the P-CCPCH is broadcast to the UEs. Thus, the UEcan measure the propagation loss during the P-CCPCH transmission basedon its transmission power. Hence, an initial target SIR(Signal-to-Interference Ratio) is set by the open loop power control.

(2) Closed Loop Power Control

The UE measures the strength, that is, an SIR of a downlink channelsignal received from the Node B. If the SIR is less than a target SIR,the UE transmits to the Node B a TPC command requesting an increase inthe transmission power of the downlink channel signal. Alternatively, ifthe SIR is equal to or greater than the target SIR, the UE transmits tothe Node B a TPC command requesting a decrease in the transmission powerof the downlink channel signal. The Node B then controls itstransmission power according to the TPC command received from the UE.

(3) Outer Loop Power Control

The above-described closed loop power control is based on an SIR.However, the quality of a radio channel signal is evaluated based on anFER (Frame Error Rate) rather than the SIR in an actual mobilecommunication system. The FER indicates an error rate limit for adigital signal required to provide good voice quality. The FER isclosely related to user satisfaction with communication quality.Therefore, an FER at which the quality of a radio channel signal ismaintained at an desirable level, that is, a target FER, is setaccording to the characteristics of a mobile communication system.

If a power control is performed in the closed loop power control methodalone, an actual FER measurement varies with channel environment despitea same SIR. As a result, the FER measurement is above or below thetarget FER, leading to an inefficient use of the entire system capacity.That is, the relation between the SIR and the FER changes irregularlydue to external factors including a channel environment and a velocityof the UE.

In this context, there is a need for a power control that maintains thetarget FER for use in the closed loop power control by changing thetarget SIR adaptively according to the channel condition. This powercontrol is an outer loop power control. The outer loop power controlmethod changes the target SIR used in the closed loop power controladaptively according to the channel condition in order to maintain aparticular performance characteristic such as the target FER.

In the CDMA mobile communication system, if the outer loop power controlis adopted, the quality of a received frame is determined by CRC (CyclicRedundancy Check). If it turns out that the frame has errors, a targetEb/No is increased by a predetermined increment unit up_step. On thecontrary, if the frame has no errors, the target Eb/No is decreased by apredetermined decrement unit down_step. The target Eb/No is used in theconcept of the target SIR herein.

The increment unit up_step and the decrement unit down_step are in therelation expressed asup_step=K×down_step   (1)The variable K is calculated by

$\begin{matrix}{K = {\frac{1}{{reqd} \cdot {FER}} - 1}} & (2)\end{matrix}$where reqd.FER is a required FER for a received frame.

As a plurality of frames are successively received duringcommunications, the FER eventually converges to the reqd.FER as notedfrom Eq. (1) and Eq. (2). As stated above, reqd.FER is a target FER fora corresponding channel. For example, if reqd.FER is 10⁻² andK×down_step is 0.5[dB], K is 99 by the above equations. When a receivedfame has errors, its transmission power is increased by 0.5[dB] and ifthe frame has no errors, the transmission power is decreased by

${\frac{0.5}{99}\mspace{11mu}\lbrack{dB}\rbrack}.$

In the outer loop power control, a received frame is CRC-checked asstated above. If the frame has no errors in the CRC check, a targetEb/No [dB] is decreased by a relatively small decrement unit down_step.On the contrary, if the frame has errors in the CRC check, the targetEB/No [dB] is increased by a relatively great increment unit. An FER inan actual channel environment converges to a target FER by adjusting theEB/No decrement unit down_step based on the Eb/No increment unitup_step.

Despite the advantage of stable maintenance of received channel signalquality in a bad channel environment, the outer loop power controlcauses excess power consumption because transmission power is decreaseda plurality of times, each time by a small decrement unit in a goodchannel environment such as indoors and increased by an increment unitgreater than the decrement unit in the bad channel environment. Thislimitation becomes serious in a mobile communication system strictlyrequiring maintenance of a target FER, especially in the good channelenvironment.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide an outerloop power control apparatus and method for setting a target Eb/Noadaptively according to channel condition in a CDMA mobile communicationsystem.

It is another object of the present invention to provide an outer looppower control apparatus and method for minimizing power consumption in aCDMA mobile communication system.

It is a further object of the present invention to provide an outer looppower control apparatus and method for performing power control reliablywith minimum power consumption under a stable channel environment in aCDMA mobile communication system.

To achieve the above and other objects, in an outer loop power controlapparatus of a mobile communication system, a raw BER (Bit Error Rate)calculation unit calculates the raw BER of a received frame. Acontroller changes a power control mode to an adjustment mode if a firstraw BER of a first received frame is equal to or greater than apredetermined threshold raw BER, decreases a target Eb/No by the productof a first step coefficient and a decrement unit if a second receivedframe following the first received frame has no errors and a second rawBER of the second received frame is less than the threshold raw BER,compares the second raw BER with the first raw BER if the secondreceived frame has no errors and the second raw BER is equal to orgreater than the threshold raw BER, and decreases the target Eb/No bythe product of the second step coefficient and the decrement unit if thesecond raw BER is less than the first raw BER.

In an outer loop power control method of the mobile communicationsystem, if a first received frame has no errors and the raw BER of thefirst received frame is equal to or greater than a predetermined rawBER, a power control mode is changed to an adjustment mode. If a secondreceived frame following the first received frame has no errors and theraw BER of the second received frame is less than the threshold raw BERin the adjustment mode, a target Eb/No is decreased by the product of afirst step coefficient and a decrement unit. If the raw BER of thesecond received frame is equal to or greater than the threshold raw BER,the raw BERs of the first and second received frames are compared. Ifthe raw BER of the second received frame is less than the raw BER of thefirst received frame, the target Eb/No is decreased by the product of asecond step coefficient and the decrement unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a block diagram of an outer loop power control apparatusaccording to an embodiment of the present invention;

FIG. 2 is a flowchart illustrating an outer loop power control operationin a normal mode according to the embodiment of the present invention;and

FIG. 3 is a flowchart illustrating an outer loop power control operationin an adjustment mode according to the embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be described hereinbelow with reference to the accompanying drawings. In the followingdescription, well-known functions or constructions are not described indetail since they would obscure the invention in unnecessary detail.

Upon generation of user data to be transmitted, a Node B on atransmitting side adds a CRC to the user data. After segmenting theCRC-attached data into code blocks for error correction, the Node Bchannel-encodes the code blocks. The encoded data blocks arerate-matched according to the length of a physical layer frame and aspreading factor (SF), for transmission to a physical layer. The ratematching is the process of adjusting the rate of the data blocks to thelength of the physical layer frame by puncturing and repetition. Therate-matched data is interleaved to prevent burst errors. Theinterleaved data is segmented into radio frames and then transmitted toa UE.

FIG. 1 is a block diagram of an outer loop power control apparatusaccording to an embodiment of the present invention. Referring to FIG.1, the UE receives the signal from the Node B at a decoder 111 and adelay 113. For clarity of description, it is assumed that the signal isreceived on a frame basis. The decoder 111 decodes the received frame ina decoding method corresponding to a channel encoding method used in theNode B. A CRC checker 115 checks the CRC of the decoded data. An encoder117 encodes the CRC-checked data in the channel encoding method andfeeds the encoded data to a raw BER (Bit Error Rate) calculator 119. Thedelay 113 delays the received frame by a predetermined time, that is,time required for processing in the decoder 111, the CRC checker 115,and the encoder 117, and feeds the delayed signal to the raw BERcalculator 119. The raw BER calculator 119 calculates a raw BER usingthe data received from the encoder 117 and the delay 113. The raw BER isdefined as the symbol error rate of a frame before channel decoding.

A controller 121 selects a power control mode between a normal mode andan adjustment mode according to the CRC check result and the comparisonresult between the raw BER and a threshold raw BER stored in a memory(not shown). In the normal mode, transmission power is controlledthrough control of a target Eb/No in the same manner as the conventionalouter loop power control. In the adjustment mode, transmission power iscontrolled through control of the target Eb/No according to the raw BER.The threshold raw BER is set by adding some margin to a raw BER for theworst case where a received frame can be recovered only if it has noerrors.

FIG. 2 is a flowchart illustrating an outer loop power control operationin the normal mode according to the embodiment of the present invention.Referring to FIG. 2, the decoder 111 decodes a received frame in adecoding method corresponding to a channel encoding method used in theNode B. The CRC checker 115 CRC-checks the decoded data and outputs theCRC-check result. The controller 121 receives the CRC-checked data fromthe CRC checker 115 in step 211 and determines whether the received datahas frame errors according to the CRC-check result in step 213.

If the received frame has errors, which indicates a bad channelenvironment, the controller 121 increases a target EB/No set for outerloop power control by a predetermined increment unit up_step, that is,K×down_step as in the conventional outer loop power control, in step215.

As described above, if the outer loop power control is adopted in theCDMA mobile communication system, the quality of a received frame isdetermined by CRC. If it is determined that the frame has errors, atarget Eb/No is increased by the predetermined increment unit up_step.Conversely, if the frame has no errors, the target Eb/No is decreased bythe predetermined decrement unit down_step. The target Eb/No is used inthe concept of the target SIR herein. The increment unit up_step and thedecrement unit down_step are in the relation that up_step=K×down_stepand

$K = {\frac{1}{{reqd} \cdot {FER}} - 1.}$Here, reqd.FER is a required FER for the received frame, that is, atarget FER.

In step 213, if the received frame has no errors, which indicates a goodchannel environment, the controller 121 decreases the target Eb/No bythe decrement unit down_step in step 217. Meanwhile, after the CRCcheck, the encoder 117 encodes the received frame in the channelencoding method used in the Node B. The raw BER calculator 119calculates a raw BER by comparing the channel encoded data with a framedelayed by a predetermined time and feeds the raw BER to the controller121. In step 219, the controller 121 compares the raw BER with apredetermined threshold raw BER. If the raw BER is less than thethreshold raw BER, the controller 121 ends the control operation. As theraw BER is less, the channel state is better. Thus, if the raw BER isless than the threshold raw BER, this indicates a good channelenvironment.

If the raw BER is equal to or greater than the threshold raw BER, whichimplies that the channel state is bad and thus even though the currentframe has no errors, a frame error probability will increase if thetarget Eb/No is maintained low, the controller 121 transitions to anadjustment mode in step 221.

In the above outer loop power control illustrated in FIG. 2, when theraw BER is equal to or greater than the threshold raw BER during thenormal mode operation, the power control mode is changed to theadjustment mode in order to adjust the Eb/No decrement.

FIG. 3 is a flowchart illustrating an outer loop power control operationin the adjustment mode according to the embodiment of the presentinvention. In the same manner as described referring to FIG. 2, thedecoder 111 decodes a received frame in the decoding methodcorresponding to a channel encoding method used in the Node B. The CRCchecker 115 CRC-checks the decoded data and outputs the CRC-checkresult. The controller 121 receives the CRC-checked data from the CRCchecker 115 in step 311 and determines whether the received data hasframe errors according to the CRC-check result in step 313.

If the received frame has errors, which indicates a bad channelenvironment, the controller 121 increases the target EB/No by theincrement unit up_step, that is, K×down_step, in step 315 andtransitions the outer loop power control mode from the adjustment modeto the normal mode in step 317. Then the controller 121 ends the controloperation.

If the received frame has no errors, which indicates a good channelenvironment, the controller 121 goes to step 319. Meanwhile, after theCRC check, the encoder 117 encodes the received frame in the channelencoding method used in the Node B. The raw BER calculator 119calculates a raw BER by comparing the channel encoded data with thedelayed frame and feeds the raw BER to the controller 121. In step 319,the controller 121 compares the raw BER with the threshold raw BER. Ifthe raw BER is less than the threshold raw BER, the controller 121 goesto step 321.

In step 321, the controller 121 decreases the target Eb/No byw₁×down_step. Step 321 is termed “A state”. While the target Eb/No isdecreased by the decrement unit down_step in the normal mode, it isfurther decreased by w₁ in the adjustment mode because a more stablepower control is possible in the adjustment mode. w₁ is defined as “afirst step coefficient”. The first step coefficient w₁ is initially setto 1 in the adjustment mode. When A state is reached continuously inframes following the current frame, the first step coefficient w₁ isincreased gradually by a predetermined first step coefficient function.

If the raw BER is equal to or greater than the threshold raw BER in step319, the controller 121 compares the raw BER of the current frame withthat of a previous frame in step 323. If the raw BER of the currentframe is equal to or greater than that of the previous frame, whichimplies that the current channel state is worse than the previouschannel state, the controller 121 increases the target Eb/No byw₂×down_step in step 325. w₂ is defined as “a second step coefficient”.The second step coefficient w₂ is initially set to 1 in the adjustmentmode. When step 325 is reached continuously in frames following thecurrent frame, the second step coefficient W₂ is decreased gradually bya predetermined second step coefficient function.

If the raw BER of the current frame is less than that of the previousframe, which implies that the current channel state is better than theprevious channel state, the controller 121 decreases the target Eb/No byw₂×down_step in step 327. The second step coefficient w₂ is initiallyset to 1 in the adjustment mode and when step 327 is reachedcontinuously in frames following the current frame, the second stepcoefficient w₂ is decreased gradually by the second step coefficientfunction. Steps 325 and 327 are termed “B state”.

While it has been described that the outer loop power control isperformed in the UE, that is, in the context of uplink TPC, it isobvious that the Node B can control a target Eb/No according to the rawBER. In this case, the UE transmits the raw BER measurement to the NodeB on a predetermined channel. The Node B then sets the target Eb/No inthe same manner as the controller 121 operates and transmits the targetEb/No to the UE on a predetermined channel. The UE updates its targetEb/No with the received target Eb/No.

In accordance with the present invention, a different target Eb/No isadaptively set in a normal mode or an adjustment mode according tochannel condition in an outer loop power control of a CDMA mobilecommunication system. Therefore, power control efficiency is increased,power consumption is minimized, and system capacity is increased.

While the invention has been shown and described with reference to apreferred embodiment thereof, it will be understood by those skilled inthe art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the invention as definedby the appended claims.

1. An outer loop power control apparatus in a mobile communicationsystem, comprising: a raw BER (Bit Error Rate) calculation unit forcalculating a raw BER of a received frame; and a controller for: (a) ifa raw BER of a present received frame is greater than or equal to athreshold raw BER, decreasing a target energy to noise ratio (Eb/No) bya product of a first step coefficient and a decrement unit, (b) if theraw BER of the present received frame is less than the threshold rawBER, determining if the raw BER of the present received frame is greaterthan or equal to a raw BER of a previous received frame, and (c) if theraw BER of the present received frame is less than the raw BER of theprevious received frame, decreasing the target energy to noise ratio(Eb/No) by a product of a second step coefficient and the decrementunit.
 2. The outer loop power control apparatus of claim 1, wherein ifthe raw BER of the present received frame is greater than or equal toraw BER of the previous received frame, the controller increases thetarget Eb/No by the product of the second step coefficient and thedecrement unit.
 3. The outer loop power control apparatus of claim 1,wherein if the present received frame has errors, the controllerincreases the target Eb/No by an increment unit.
 4. The outer loop powercontrol apparatus of claim 1, wherein the raw BER calculation unitcomprises: a delay for delaying a received frame by a predeterminedtime; an encoder for encoding the received frame after a CRC check; anda raw BER calculator for calculating the raw BER by comparing thedelayed frame with the encoded frame.
 5. The outer loop power controlapparatus of claim 1, wherein if the controller continuously decreasesthe target Eb/No by the product of the first step coefficient and thedecrement unit in frames following the present received frame, thecontroller gradually increases the first step coefficient for each ofthe following frames.
 6. The outer loop power control apparatus of claim5, wherein the first step coefficient is initially set to
 1. 7. Theouter loop power control apparatus of claim 1, wherein if the controllercontinuously decreases the target Eb/No by the product of the secondstep coefficient and the decrement unit in frames following the presentreceived frame, the controller gradually decreases the second stepcoefficient for each of the following frames.
 8. The outer loop powercontrol apparatus of claim 7, wherein the second step coefficient isinitially set to
 1. 9. The outer loop power control apparatus of claim1, wherein if the controller continuously increases the target Eb/No bythe product of the second step coefficient and the decrement unit inframes following the present received frame, the controller graduallydecreases the second step coefficient for each of the following frames.10. The outer loop power control apparatus of claim 9, wherein thesecond step coefficient is initially set to
 1. 11. An outer loop powercontrol method in a mobile communication system, comprising the stepsof: if a present received frame has no errors, comparing a raw BER (BitError Rate) of the present received frame with a threshold raw BER; ifthe raw BER of the present received frame is greater than or equal tothe threshold raw BER, decreasing a target energy-to-noise ratio (Eb/No)by a product of a first step coefficient and a decrement unit; if theraw BER of the present received frame is less than the threshold rawBER, comparing the raw BER of the present received frame with a raw BERof a previously received frame preceding the present received frame; andif the raw BER of the present received frame is less than the raw BER ofthe previously received frame, decreasing the target Eb/No by theproduct of a second step coefficient and the decrement unit.
 12. Theouter loop power control method of claim 11, further comprising the stepof increasing the target Eb/No by the product of the second stepcoefficient and the decrement unit if the raw BER of the presentreceived frame is greater than or equal to raw BER of the previouslyreceived frame.
 13. The outer loop power control method of claim 12,further comprising the step of gradually decreasing the second stepcoefficient for each of frames following the present received frame ifthe target Eb/No is continuously increased for the following frames bythe product of the second step coefficient and the decrement unit. 14.The outer loop power control method of claim 13, wherein the second stepcoefficient is initially set to
 1. 15. The outer loop power controlmethod of claim 11, further comprising the step of increasing the targetEb/No by an increment unit if at least one of the previously receivedframe and the present received frame has errors.
 16. The outer looppower control method of claim 11, further comprising the step ofgradually increasing the first step coefficient for each of framesfollowing the present received frame if the target Eb/No is continuouslydecreased for the following frames by the product of the first stepcoefficient and the decrement unit.
 17. The outer loop power controlmethod of claim 16, wherein the first step coefficient is initially setto
 1. 18. The outer loop power control method of claim 11, furthercomprising the step of gradually decreasing the second step coefficientfor each of frames following the present received frame if the targetEb/No is continuously decreased for the following frames by the productof the second step coefficient and the decrement unit.
 19. The outerloop power control method of claim 18, wherein the second stepcoefficient is initially set to 1.